1. Field of the Invention
The present invention relates to the field of communications, and more particularly, the present invention relates to ultrawide-band impulse communication systems and methods employing subcarriers.
2. Background Art
Designers of radio technology for personal communications devices, medical and military devices, and the like, are currently faced with several development challenges. Low power consumption, reuse of available spectrum, channelization and cost are four of the main issues.
These issues are addressed in part by an emerging, revolutionary technology called impulse radio communications (hereafter called impulse radio). Impulse radio was first fully described in a series of patents, including U.S. Pat. Nos. 4,641,317 (issued Feb. 3, 1987), 4,813,057 (issued Mar. 14, 1989), 4,979,186(issued Dec. 18, 1990) and 5,363,108 (issued Nov. 8, 1994), all to Larry W. Fullerton. These patent documents are incorporated herein by reference.
Basic impulse radio transmitters emit short Gaussian monocycle pulses with tightly controlled average pulse-to-pulse interval. Impulse radio systems use pulse position modulation. Pulse position modulation is a form of time modulation in which the value of each instantaneous sample of a modulating signal is caused to modulate the position in time of a pulse.
For impulse radio communications, the pulse-to-pulse interval is varied on a pulse-by-pulse basis by two components: an information component and a pseudo-random code component. Spread spectrum systems make use of pseudo-random codes to spread the normally narrowband information signal over are latively wide band of frequencies. A spread spectrum receiver correlates these signals to retrieve the original information signal. Unlike spread spectrum systems, the pseudo-random code for impulse radio communications is not necessary for energy spreading because the monocycle pulses themselves have an inherently wide information bandwidth (information bandwidth, hereafter called bandwidth, is the range of frequencies within which performance, with respect to some characteristics, falls within specific limits). Instead, the pseudo-random code is used for channelization, energy smoothing in the frequency domain, and jamming resistance.
The impulse radio receiver is a homodyne receiver with a cross correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. (The baseband signal is the basic information channel for the basic impulse radio communications system, and is also referred to as the information bandwidth.) The data rate of the impulse radio transmission is only a fraction of the periodic timing signal used as a time base. Each data bit time position modulates many pulses of the periodic timing signal. This yields a modulated, coded timing signal that comprises a train of identical pulses for each single data bit. The cross correlator of the impulse radio receiver integrates multiple pulses to recover the transmitted information.
As with all aspects of the electronics field, what is desired are still smaller, lower power and more flexible systems. However, generally accepted principles in continuous wave (CW) radio technology do not readily lend themselves to time domain systems, such as impulse radio.
Descriptions of some of the basic concepts discussed below are found in a number of references, including Robert C. Dixon, Spread Spectrum Systems (John Wiley & Sons, Inc., New York, 1984, 2nd ed.); and Don J. Torrieri, Principles of Military Communication Systems (Artech House, Inc., Dedham Mass., 1982, 3rd ed.).